This invention relates to an apparatus for estimating direction of arrival wave, such as radio wave and acoustic wave, which arrives at a plurality of antenna elements.
A technique of estimating arrival directions of one or more arrival waves, such as radio waves and acoustic waves, of the same frequency which are caught by two or more antenna elements is well known. As one of such methods, MUSIC (Multiple Signal Classification) method is well known (see Japanese patent application the publication number of which is 2000-121716.
In MUSIC method, on the basis of a signal data vector of arrival waves, a correlation matrix thereof is obtained, and an eigenvalue decomposition operation for decomposing the obtained correlation matrix into eigenvectors and eigenvalues is implemented, and azimuths of the arrival waves are estimated from the thus obtained eigenvectors. In order to obtain the arrival azimuth, it is necessary to estimate the number of arrival waves in advance. Generally, eigenvalues are used in the estimation of the number of arrival waves. Such methods are AIC, MDL and threshold methods, for instance.
[Patent-related document 1] Japanese patent application the publication number of which is 2000-121716
In such an eigenvalue decomposition operation, methods with Householder's method or Jacobi method are known. If a Signal to Noise ratio of an arrival wave is high, that is, the arrival wave is very big in comparison with the noise, or a difference of arrival azimuths of two or more arrival waves is small, the ratio of a maximum value and a minimum value of the eigenvalues is high, so that underflow may occur in the operation of the smaller eigenvalue, and the computed eigenvalue may be a negative value or a value credibility on which is low. If a fixed decimal mode is used for a word size which is insufficient for an operation, the above-mentioned inconvenience is especially ready to appear.
If the credibility on the eigenvalues is low, the number of arrival waves which is estimated from the eigenvalues is likely to be incorrect. In addition, the vain operation is repeated in the eigenvalue decomposition operation in order to finally obtain the eigenvalue having low credibility, so that unnecessary operation is loaded on a processing unit.
The object of the invention is to provide an apparatus for estimating direction of arrival wave, for reliably estimating the number of arrival waves, extremely avoiding vain eigenvalue decomposition operations in order to overcome the above-mentioned inconvenience.